This invention relates to integrated optical Mach Zehnder structures, and in particular to such structures constructed in integrated optical waveguide format. Such a format typically, but not necessarily, comprises a silicon substrate upon which has been grown, in succession, a dielectric layer of buffer material, a waveguide layer of higher refractive index material, and a cladding layer which is not deposited until after the waveguide layer has been patterned and selectively etched to produce a pattern of ribs (cores) of waveguide layer material.
The invention is particularly concerned with the regulation of birefringence in integrated optical waveguide format dynamically controllable Mach Zehnders, and is also concerned with the use of such Mach Zehnders as wavelength selective controllable attenuators. Birefringence is significant in such Mach Zehnders because such birefringence has the effect of making the optical attenuation exhibited by such a structure different for the two principal states of polarisation (principal SOPs).
The integrated optical waveguides that were initially produced using silica on single crystal silicon technology exhibited significant birefringence which has been attributed to the effects of anisotropic strain resulting from thermal expansion coefficient mismatch between that of the material of the waveguide cores and that of the underlying silicon substrate. More recently, it has been possible to produce integrated optical waveguides with much reduced birefringence. One method for producing such low birefringence integrated optical waveguides involves choosing for the material of the cladding layer a material having a thermal expansion coefficient matched with that of the material of the substrate, as is described in the specifications of U.S. patent application Ser. No. 081942189, and its European counterpart, European Patent Application No. 98306084.9.
The use of thermo-optic phase-shifters to adjust the relative optical path length of the two interference arms of an integrated optical waveguide Mach Zehnder is known and is, for instance, described in the invited paper by Masao Kawachi entitled, `Silica waveguides on silicon and their applications to integrated-optic components`, Optical and Quantum Electronics, Vol. 22, (1990) pp391-416. Such a phase-shifter employs a thin-film Joule-effect heating strip on the cladding layer above and in registration with a portion of underlying waveguide core. The thermo-optic constants of the constituent parts of an integrated optical waveguide are such that, when an electric current is passed through a heating strip overlying such a waveguide, the effective refractive index of the underlying waveguide is increased. Hence the optical path length of that waveguide is increased. The heating also has a mechanical effect due to the thermal expansion coefficient mismatch between these constituent parts. This effect is to introduce stress birefringence because the locally heated material is more readily able to expand in the direction extending normal to the plane of the substrate than in any direction lying in the plane of that substrate. The magnitude of this stress-induced birefringence can be reduced by the etching of grooves into the integrated optics structure on either side of the heating strip in the same manner as is described in the above-mentioned paper for reducing the stress-induced birefringence in integrated optics waveguides employed in other types of integrated optics device.
This groove etching method of reducing the value of stress-induced birefringence in a thermo-optic phase-shifter is not susceptible of adjustment after the creation of the phase-shifter. If, for a specific integrated optics Mach Zehnder structure incorporating one or more such phase-shifters, the particular dimensions of the or each phase-shifter are found in practice not to provide adequate suppression of birefringence, there is no obvious way of remedying the situation in respect of that specific structure.